The present invention relates generally to bulk solids and more particularly to measuring cohesive, adhesive and frictional properties of bulk solids.
Measuring the cohesive, adhesive and frictional properties (sometimes called the Theological properties) of bulk solids has previously been accomplished by a split ring shear cell to which normal and shear forces are applied to a specimen of particulate solid confined between flat top and bottom disks and at the sides with top and bottom split rings that move with respect to each other. Examples of there testers include the Jenike tester and the Pechel tester. The magnitude of the normal and shear forces applied to the rings then determine the state of stress in the mass of particulates. Sometimes a tri-axial test cell is used in which a cylinder of bulk solids is subjected to a lateral pressure applied through a flexible membrane and an axial force from which the state of stress in determined. Most recently a uni-axial tester is used in which solids are consolidated uni-axially in a cylinder and then sheared along a conical surface that is coaxial with the cylinder.
Each of these testers and test methods share similar deficiencies. Each imposes a severely non-uniform stress and strain in the sample during the load application and each is difficult to interpret. For example, the Jenike type split ring shear cell develops a high stress concentration at the front of the ring being pushed by the shear load and at the back of the ring which remains stationary. This concentration is so severe that with cohesive bulk solids, a void often forms at both the front of the bottom ring and the back of the top ring of the test apparatus. This makes a proper analysis of the test results difficult at best and decreases both accuracy and precision of the test results. The split rotational ring tester causes a strain rate gradient from zero at the center to a maximum on the outer edge of the rings thus causing an ill-defined strain and stress state. The Tri-axial tester allows an uncontrolled shear plane to develop during sample failure. The uni-axial tester provides relatively uniform initial compaction. However, during failure a very non-uniform stress occurs at both the top and the bottom of the specimen.
All but the uni-axial tester are very difficult to interpret making them time consuming to run. For example, to measure the strength of a bulk solid at a single consolidation pressure with a split ring shear tester requires three to six tests to obtain enough data for proper interpretation. The tri-axial tester requires at least three cumbersome tests for a single strength value. While the uni-axial tester can produce one approximate strength value with a single test, an auxiliary test is required to obtain the internal frictional properties of the bulk solid.
There is thus a need in the art for a test apparatus and method for properties of bulk solids that provides uniform stress and strain in the sample during the load application and is uncomplicated to interpret.